The present invention relates to a circulation fan for baking ovens, particularly for combined circulation and microwave ovens. In the case of combined circulation and microwave ovens, it must be insured that the radio frequency (RF) radiation in a baking muffle does not leak out. A particularly critical point where leakage should be prevented is not only the opening for the drive shaft in the back wall of the baking muffle, but also the drive shaft itself. To achieve a certain amount of leakage damping, the diameter of the opening should not exceed one quarter of a wavelength of the radio frequency. This condition can be met without major difficulty. To control the part of the radio frequency escaping via the drive shaft is much more difficult. If, for example, the length of the portion of the drive shaft protruding from the baking muffle is the wavelength lambda or one quarter, one half, five eighths, or a multiple of the wavelength lambda times a velocity factor, the drive shaft will act as a radiator (part of an antenna), with the back wall of the baking muffle used as a counterpoise. The velocity factor is a function of the material and the diameter of the drive shaft. Part of the energy radiated via the draft shaft is damped and converted into heat by the stator lamination of the drive motor and by the supporting members, but the free part can radiate unhindered. From German Pat. No. 31 18 463 it is known that a drive shaft made from ceramic material prevents the leakage of RF energy. This solution is very effective but also very expensive .